This invention relates to the generation of electricity and, more particularly, to the generation of electricity by a combination water and compressed air storage system.
An underground hydro-pumped storage project may be of interest where topography precludes the above-ground construction of two reservoirs of suitable size, difference in elevation and proximity to each other. An underground lower reservoir can then take the place of one above-ground reservoir, if suitable geology makes the required excavation possible and economic. Because that part of the output capacity related to a given volume of excavation is a direct function of the developed head (difference in elevation), the under-ground reservoir will usually be placed as deep as plant equipment and optimized design will permit.
In a hydro-pumped storage system for intermittently generating electricity, such as that described in U.S. Pat. No. 1,247,520, a surface water reservoir is connected by a vertical conduit to an excavated subterranean power plant. Water from the surface reservoir flows through turbines of the power plant and electricity is produced. After leaving the turbines the water is stored in an excavated subterranean reservoir. An air vent is usually provided to allow air to flow to and from the underground reservoir to equalize the pressure. Also, the reservoir is normally located above the generator in order to avoid cavitation and to assure satisfactory operation of the turbines and pumps. Later, wind or electricity during low demand periods can be used to pump the water back to the surface reservoir. This removal of the water from the subterranean reservoir draws air down the vent to replace the water.
Constant pressure stored-air systems, such as the one described in U.S. Pat. No. 3,538,340, may also be used for the intermittent supply of electricity. In this type of system, water from a surface reservoir is connected to the underside of an excavated subterranean reservoir, which contains compressed air. A shaft connects the compressed air at the top of the subterranean reservoir with an air motor or other auxiliary prime mover which, in turn, drives generating equipment to produce electricity. As the compressed air in the subterranean reservoir is forced to the surface and is used in a turbine to drive an electric generator, a constant pressure is maintained on the air by water entering the subterranean reservoir from the surface reservoir. Thus, the air pressure in the subterranean reservoir is maintained essentially constant and is determined by the hydrostatic head, i.e. difference in elevation, between the surface reservoir and the subterranean reservoir. During low demand periods electrically driven air compressors at the surface deliver compressed air to the subterranean reservoir. This compressed air displaces the water and forces it to return to the surface reservoir.
In U.S. Pat. No. 2,433,896 a combined water and air storage system is described. In this system there is a first subterranean reservoir for storing combustible gas and a second subterranean reservoir for storing air. The surface reservoir is connected through a conduit to a pump-turbine also located on the surface. Other conduits lead from the pump-turbine to the bottom portions of the two subterranean reservoirs. Conduits also lead from the top of the two subterranean reservoirs to air compressors and air turbines located on the surface. During periods of high load, the water pump is activated to pump water from the surface reservoir into the two subterranean reservoirs. This causes air to be forced out of one subterranean reservoir and gas to be forced out of the second subterranean reservoir, whereupon they are combined and injected into a combustion turbine to drive an electrical generator. During periods of low load, the air compressors compress the air and gas and inject them into their respective subterranean reservoirs. Water in the subterranean reservoirs is forced through the hydrostatic head to the surface reservoir. In passing from the subterranean reservoirs to the surface reservoir the water passes through a turbine for generating electrical power.
The disadvantage with this combined system is that only the combustion turbine generates electricity during peak periods, and during low load periods the air compressors must use enough power to recharge the subterranean reservoirs and force the water to the surface reservoir through a turbine, which generates electricity at a time when it is less needed.